Foaming is a common problem in many industrial processes, particularly in paper making operations where foam can prevent proper formation of the finished paper and disrupt manufacturing operations. Defoamers, sometimes called anti-foaming agents, can be added to process streams to combat this problem. Commonly used agents are insoluble oils, polydimethylsiloxanes and other silicones, certain alcohols, stearates and glycols. The defoamer can be added either to prevent formation of foam or to break a foam already formed. Rainer Höfer et al., “Foams and Foam Control,” Ullmann's Encyclopedia of Industrial Chemistry, 2000, Wiley-VCH, Weinheim.
Generally a defoamer is insoluble in the industrial process stream to which it is added. Defoamers typically have a low viscosity, surface-active properties, and the ability to spread rapidly on foamy surfaces. They also have affinity for the air-liquid surface where they destabilize the foam lamellas. This causes the air bubbles to rupture and the surface foam to break down. Entrained air bubbles agglomerate, and the larger bubbles rise to the surface of the bulk liquid more quickly. Martin Hubbe, NC State University. “Mini-Encyclopedia of Papermaking Wet-End Chemistry.” Accessed Dec. 7, 2014.
Experiments with silicone-based defoamers started during the 1950s. These were based on polydimethylsiloxane (silicone oil) dispersed in water or light oil. Silicone oils worked well, but caused surface disturbances in many applications like paints and papermaking. In 1963, the first defoamers made with hydrophobic silica particles in light oil were patented, including dispersions of silica particles in silicone oil (so-called “filled silicone”). These eventually were replaced by water-based defoamers, and today the predominant defoamers in the paper-making industry use filled silicone present as an oil-in-water emulsion.
Surfactants are commonly used to stabilize these silicone-in-water emulsions and thereby prevent them from separating into two liquid phases. The surfactants both stabilize the emulsion, and participate in the defoaming process by facilitating the spread of the defoaming agent at the air-liquid interface.
The manufacture of silicone-based defoamers is a time and heat intensive process, primarily because of the need to melt the surfactants and other defoamer ingredients before forming an emulsion. What is needed are improved techniques and compositions for manufacturing silicone emulsion defoamer compositions that require less time and use less energy for production. Ideally, the resulting emulsion would have improved anti-foaming activity, improved finished product performance, and improved activity in the process stream to which it is added.